As it turns out, part of defining a new type is the
definition of functions that describe its behavior.
Consequently, while it is possible to define a new function
without defining a new type, the reverse is not true. We
therefore describe how to add new functions to PostgreSQL before describing how to add
new types.

Every kind of function can take a base type, a composite
type, or some combination as arguments (parameters). In
addition, every kind of function can return a base type or a
composite type. It's easiest to define SQL functions, so we'll start with those.
Examples in this section can also be found in funcs.sql and funcs.c
in the tutorial directory.

Throughout this chapter, it can be useful to look at the
reference page of the CREATE FUNCTION
command to understand the examples better.